Digital-to-time converters (DTC) are used to convert digital information to a time information or a phase shift. This time information or phase shift can be used for generating a frequency- or phase-modulated clock in a transmitter.
Digital-to-time converters have become attractive because in the last years it has become possible to implement or realize RFDACs (RFDAC=radio frequency digital-to-analog converters) which are able to directly transfer digital data to the RF (RF=radio frequency) domain. This RFDAC can be operated in IQ (I=in-phase; Q=quadrature) or polar mode.
In IQ mode a fixed clock can be used and the phase modulation is done by adding two orthogonal vectors. No extra frequency or phase modulator is needed, but a drawback of this method is a loss of square root of two of efficiency when the two vectors have to be summed together.
More power efficient is the polar mode where the amplitude modulation is done by an RFDAC and the phase information is delivered by modulating a PLL (PLL=phase locked loop). This polar mode works fine for low baseband signal bandwidth, but for modern wireless standards like LTE (LTE=long term evolution) bandwidth becomes 40 MHz and more. This bandwidth is too high for modulating a PLL.